I haven't looked this up extensively, but I think the case is yes, you can put 2 sticks of 2gb ram, totaling 4gb. BUT the Macbook maxes out it's usage at 3.4gb.

So why not put in 2gb + 1gb in? Well supposedly ram runs best when the pairs of ram match, while non-matching ram is not as efficient. But I've seen one vendor claim that putting in 2+1 has a negligible difference than putting in 2+2, so cost-wise this seems to be the best solution. At least that's what I've gathered so far, and I'll probably put in 2+1 down the road, when Leopard or Logic 8 comes out, whatever comes first.

The maximum amount of usable ram in a MacBook C2D is 3gb. It can handle 4gb and will be slightly faster with 4gb, but it will only recognize 3gb. OWC sells a 3gb kit for $158. Check out this review for more information:

The maximum amount of usable ram in a MacBook C2D is 3gb. It can handle 4gb and will be slightly faster with 4gb, but it will only recognize 3gb. OWC sells a 3gb kit for $158. Check out this review for more information: